Method of encoding and decoding

ABSTRACT

The invention relates to a method of encoding user data into codevectors and to a corresponding method of decoding codevectors into user data. In order to be able to use the same ECC decoder for decoding of more than one type of data a method of encoding is proposed comprising the steps of: generating a first block of a fixed first number of data symbols by taking a fixed second number, being smaller than said first number, of user data symbols, and a fixed third number of dummy data symbols, and by arranging said user data symbols and said dummy data symbols in a predetermined order, 
         encoding said first block of data symbols using an ECC encoder ( 2 ) to obtain a codeword having a fixed number of symbols, said codeword comprising said first block of data symbols and a second block of a fixed forth number of parity symbols, and generating a codevector by selecting a fifth predetermined number of user data symbols and a sixth predetermined number of parity symbols from said codeword, the sum of said fifth and sixth number being predetermined and smaller than the sum of said second and forth number.

The invention relates to a method of encoding user data into code wordsof an error correcting code (ECC), to a corresponding method of decodingcode words of an error correcting code into user data, to correspondingdevices for encoding or decoding, to an information carrier and to acomputer program product.

Information carriers like rewritable optical discs, such as a CD-RW, aDVD+RW or a DVR information carrier, contain different kinds of data.For example, a rewritable optical record carrier comprises written userdata like video or audio information in the phase change material andaddress information, for example specifying the position of the userdata in each field, the track number, the frame number, the field numberor the line number, in the wobble channel. To protect this informationparities are added to the information in such a way that errors duringread out can be corrected. A well-known method to calculate and correctdata with parities are error correcting codes, particularly Reed SolomonCodes (RS codes).

In a reading device for reading information from an information carrierparticularly the costs for the hardware of the decoder, i.e. the errorcorrecting unit, are high. When due to careful design of the errorcorrecting code used for storing data on the information carrier,however, it will be possible to use the same decoder for more than onetype of data so that hardware costs for different types of decoders inone reading device can be saved. However, different types of data almostalways imply different types of constraints such as block length andparity length of the decoder which issues have to be solved.

The issue of different block length is already addressed in WO 01/04895A1 Therein a device for reading an information carrier carrying anidentification information and user information is disclosed. Theidentification information is arranged so as to be spread over theinformation carrier. Organization means are provided for organizing theinformation in such a way that both the identification information andthe user information can be processed by the error correction means.

It is an object of the present invention to provide methods of encodingand decoding as well as corresponding devices which enable the use ofthe same decoder for different types of data, particularly errorcorrecting codes having different numbers of parities.

This object is achieved according to the present invention by a methodof encoding as claimed in claim 1, comprising the steps of:

-   -   generating a first block of a fixed first number of data symbols        by taking a fixed second number, being smaller than said first        number, of user data symbols, and a fixed third number of dummy        data symbols, and by arranging said user data symbols and said        dummy data symbols in a predetermined order,    -   encoding said first block of data symbols using an ECC encoder        to obtain a codeword having a fixed number of symbols, said        codeword comprising said first block of data symbols and a        second block of a fixed forth number of parity symbols, and    -   generating a codevector by selecting a fifth predetermined        number of user data symbols and a sixth predetermined number of        parity symbols from said codeword, the sum of said fifth and        sixth number being predetermined and smaller than the sum of        said second and forth number.

A corresponding method of decoding codevectors according to the presentinvention is claimed in claim 2, comprising the steps of:

-   -   generating a codeword comprising said fixed third number of        dummy data symbols, a codevector and a seventh number of filling        symbols, arranged in a predetermined order, the sum of said        third, fifth, sixth and seventh number being equal to said the        sum of said first and forth number,    -   decoding said codeword using an ECC decoder to obtain said user        data symbols embedded in said codevector.

The present invention is based inter alia on the idea to define a firstblock having a fixed block length, to fill in user data to be encoded inone portion and to fill up the remaining portion with dummy datasymbols. The block length is chosen such that it is consistent with theblock length expected by an ECC encoder already present and used forencoding other data. After encoding of said block, however, not thecomplete obtained codeword is used as codevector and, e.g. stored on aninformation carrier or transmitted over a network, but only a certainpart thereof; particularly a predetermined number of user data symbolsand parity symbols included in said codeword in order to save storageand/or to comply with given storage requirements.

Correspondingly, during decoding the same codeword is formed, filled inwith the received codevector, the same dummy data symbols and, inremaining empty portions, with filling symbols. Said filling iscontrolled such that the order of the symbols is the same as in thecodeword obtained during encoding. Thus, an ECC decoder already presentand used for decoding codevectors of other codes can be used fordecoding said codevectors to obtain the user data embedded in saidcodevectors. This simplifies devices for recording and/or reading ofinformation carriers storing different types of data because, generally,only one type of error correcting means has to be included reducing theproduction costs of such devices.

It should be noted that it is not relevant for the invention which userdata symbols and which parity symbols of a codeword are taken and usedas a codevector. Further, the position of the dummy data symbols and theuser data symbols in a codeword are arbitrary; the only requirement isthat the positions of the dummy data symbols and the user data symbolsare known and that the values of the dummy data symbols are known.

Preferred embodiments of the invention are defined in the dependentclaims. In accordance with a preferred aspect of the invention anerasure flag is used indicating to the decoder that the codewordcontains filling symbols to be corrected by said ECC decoder, inparticular indicating the position and/or the number of filling symbolsin said codeword to said ECC decoder. This has the advantage that thenumber of parities necessary to correct errors by an ECC decoder can bereduced, if the decoder already knows that there are errors and in whichpositions these errors are. E.g., when the decoder already knows thatthe codeword comprises 16 errors, i.e. comprises 16 filling symbolsmarked as erasures by erasure flags, only 16 parities are required tocorrect these errors, leaving 16 parities for correcting additionalerrors in the written codevector. Without such erasure flags, 32parities would be necessary to correct 16 errors.

The method according to the invention is preferably used for encoding ordecoding, respectively, user data to be recorded on an optical recordcarrier, particularly a CD, a CD-ROM, a DVD or a DVR disc of,preferably, a rewritable or recordable type. Particularly in the fieldof DVR user data are stored in a special purpose zone (SPZ) or a BurstCutting Area (BCA). In said zone, which is located at the most innerside of the disc, a “barcode” is written. The data in this barcode isprotected by an ECC. Since the bit density of the barcode is very lowonly 32 bytes can be stored therein. In order to protect these byteswith an ECC which has a Hamming distance of 17, i.e. which uses 16parities, the same decoder as used for decoding codewords of a longdistance codeword (LDC) or for decoding Burst Indicator Subcode (BIS)words is preferably used.

Corresponding devices for encoding and decoding, respectively, aredefined in claims 10 and 11. The invention relates also to aninformation carrier, in particular an optical recording medium, storingcodevectors of an error correcting code encoded by a method as claimedin claim 1. Still further, the invention relates to a computer programproduct comprising program code means for performing the steps of themethod as claimed in claim 1 or 2 if said computer program runs on acomputer.

The invention will now be explained in more detail with reference to thedrawings, in which

FIG. 1 shows a block diagram illustrating the methods of encoding anddecoding according to the present invention,

FIG. 2 shows the generation of a codeword and a codevector usedaccording to the present invention,

FIG. 3 shows an embodiment of an encoding apparatus illustrating codepuncturing,

FIG. 4 shows an embodiment of a decoding apparatus illustrating codepuncturing,

FIG. 5 shows another codevector according to the invention, and

FIG. 6 shows still another codevector according to the invention.

The block diagram shown in FIG. 1 illustrates the methods of encodingand decoding according to the present invention. In a block generationunit 1 a first block B of a fixed first number of data symbols isgenerated. Said block generation unit 1 receives as input a number ofuser data symbols U and a number of dummy data symbols D which arearranged in a predetermined order to form said block B. Said block B ofdata symbols is thereafter encoded by an ECC encoder 2 to obtain acodeword E, i.e. to obtain parity symbols for error correction. Whileconventionally said codewords E are completely used as codevectors,according to the present invention only a fixed portion of saidcodewords E is used as codevectors C which are stored on an informationcarrier 5 by a write unit 3 under control of a control unit 4. Saidcontrol unit 4 controls the generation of said codevectors C from saidcodewords E, i.e. selects according to a fixed rule which symbols ofsaid codewords E are used as codevectors C.

These blocks and symbols can be seen in FIG. 2 showing a completecodeword E and the different portions thereof. As explained, saidcodeword E comprises a first block B of a first fixed number Z1 of datasymbols. Said data symbols comprise a fixed second number Z2 of userdata symbols U (U1, U2) and a third fixed number Z3 of dummy datasymbols D. These dummy data symbols D are filled in, to achieve thefixed block length of said block B and can, in general, be freelychosen. Preferably they are chosen as non-zero values, particularlyhaving the value FF in hexadecimal notation. The ECC encoder 2calculates a fourth fixed number Z4 of parity symbols P (P1, P2)resulting in an encoded codeword E having in total Z1+Z4 symbols.Therefrom codevectors C are generated by selecting a fifth fixed numberZ5 of data symbols U2 and a fixed sixth number Z6 of parity symbols P1.Said codevectors C are then stored on the record carrier 5.

To give a more detailed example which may be applied for storing data ona DVR information carrier, particularly to protect data to be stored ina barcode of the burst cutting area (BCA) of a DVR information carrierthe first block B will be formed by 16 user data symbols U and 14 dummydata symbols D, thus coming to 30 data symbols of the first block B. ThePIC and main data of a DVR information carrier include so-called BIS(Burst Indicator Subcode) data which are protected by a RS code with 32parities and having a codeword length of 62, i.e. being protected by a(62, 30, 33) RS code. In order to be able to use an ECC decoder to bebuilt for said code also for decoding the user data stored in thebarcode of the BCA the first block B having 30 data symbols is encodedby a corresponding ECC encoder, i.e. an encoder for a [62, 30, 33] code,generating 32 parity symbols, resulting in a block length of 62 symbolsof the codeword E. Since the bit density of the barcode in the BCA isvery low only 32 symbols (bytes) can be stored therein. Thus, accordingto the present invention, from said codeword E the 16 user data symbolsU and 16 parity symbols P are used as codevector C and stored on theinformation carrier. However, in general the method according to theinvention will also work if less user data symbols and more paritysymbols are combined to form a codevector C as long as the sum of saidsymbols is 32. In the embodiment shown in FIG. 2 a number Z5 of userdata symbols U2, e.g. 12 user data symbols U2, and a number Z6 of paritysymbols P1, e.g. 20 parity symbols P1, are combined into one codevector.

It should be noted that it does not matter which symbols of the U and Pportions of the codeword E are taken and used as codevector C. Further,the position of the D and U portions in the codevector C are arbitrary.The positions can be swapped (first U and then D); the only requirementis that the positions for the U and D portions are known and that thevalues of the D symbols are known.

During decoding the codevectors C are read from the information carrier5 by a reading unit 6 and further inputted into a codeword generationunit 7. Therein the codeword E will be regenerated so that it has thesame number and arrangement of symbols as during encoding. Therefore,the codeword E is filled with said third number Z3 of dummy data symbolsD having the same value as the dummy data symbols D used duringencoding. Thereafter the codevector C including said fifth number Z5 ofuser data symbols U2 and said sixth number Z6 of parity symbols P1 areinserted at the same positions as they have been in the codeword duringencoding. Finally, remaining portions are filled with filling symbolsF1, F2, i.e. a seventh number (Z71+Z72) of filling symbols F1, F2 isfilled in at positions where in the codeword E during encoding user datasymbols U1 and parity symbols P2 had been located, but had not beenstored on the information carrier 5. The filling of said codeword canpreferably be achieved by sending the data thereof in the correct orderto an ECC decoder 8 adapted to decode such codewords E to obtain theoriginal user data U comprising the user data symbols U1 and U2.

To enable the codeword generation unit 7 to reconstruct the codeword Eit must be known to said unit 7 how the codeword E had been constructedduring encoding, i.e. the number of dummy data symbols D, user datasymbols U and parity symbols P, their positions in the codeword E aswell as the length of the codevector including the positions of symbolsselected to form said codevector C have to be known to the codewordgeneration unit 7, e.g. have to be fixed by a corresponding standard.Also the value of the dummy data symbols D have to be fixed in advance.

Reverting to the above described example for storing data in the barcodeon an DVR information carrier, where the codevector C comprises 12 userdata symbols U2 and 20 parity symbols P1, it will be clear that 4 (Z71 )filling symbols F1 and 12 (Z72) filling symbols F2 are filled into theremaining portions during decoding to form the codeword E.

Preferably, the filling symbols are flagged as erasures so that the ECCdecoder only requires Z71+Z72 parities to correct these errors. In theexample, only 16 parities are needed to correct said 16 errors (fillingsymbols), similar to a conventional 16 parity code which leaves 16parities to correct errors in the written codevector which is similar toa conventional 16 parity RS code, while without such erasure flags twiceas many parities would be needed for a correction.

As already mentioned above the number Z5 of user data symbols U2 and thenumber Z6 of parity symbols P1 used to form the codevector C are notfixed, but only the sum Z5+Z6 of said numbers is fixed. Thus it may alsobe possible to use no user data symbols U and all parity symbols P, i.e.Z4 parity symbols, as codevector C. During decoding, at first Z3 dummydata symbols D, thereafter Z2 filling symbols F and finally Z4 paritysymbols would then be sent as codeword E to the ECC decoder to obtainthe Z2 user data symbols U, which have originally been located at thepositions of the filling symbols F. Also in this case the Z2 user datasymbols (erasures) can be calculated using Z2 (being smaller than Z4)parity symbols and using the remaining Z4-Z2 parity symbols to correcterrors from the information carrier.

If a conventional 16 parity RS code is used 16 data symbols and 16parities are usually written on a disc. In this codeword of 32 symbols amaximum of 16 errors can be corrected. According to the presentinvention a 32 parity RS code is used which will offer the sameperformance of the 16 parity RS code. It is important to note thataccording to the invention the codevector, e.g. the symbols written ondisc, belong to a 32 parity RS codeword and can not be decoded by a 16parity RS decoder. When applying the invention in DVR, on the encodingside a 248 symbols codeword is formed which comprises 200 dummy datasymbols, 16 user data symbols and 32 parity symbols, i.e. a (248, 216,33) RS code is used, called LDC or Long Distance Code in DVR. From the16 user symbols and 32 parity symbols 32 symbols are written to disc ascodevector. Again, it is important to mention that is does not matterwhich 32 from these 48 symbols are written to disc. On the decoding sidethe same 248 symbol codeword is formed. The 200 known dummy data symbolsare placed on the correct positions in the codeword. The 32 symbolswritten to disc are also placed in the codeword and the 16 non written(and unknown) symbols are passed to the decoder as erasures. The decoderuses 16 of the 32 parities to calculate the 16 unknown symbols whichleaves 16 parities to correct errors in the 32 symbol writtencodevector. Thus, a performance can be achieved as if a 16 parity RScode was used.

The general use of code puncturing, as particularly described inEuropean patent application EP 01201841.2, the description of which isherein incorporated by reference, shall now be explained with referenceto FIGS. 3 and 4. FIG. 3 illustrates the method of encoding aninformation word m into a codeword c and FIG. 4 illustrates the methodof decoding a possibly mutilated codeword r into an information word m.

As shown in FIG. 3 the information word m comprising k informationsymbols is encoded by an encoding unit 41 of an encoding apparatus 40using an intermediate generator matrix G″. Said intermediate generatormatrix G″ derives from a generator matrix G which has been selected by aselection unit 42 as particularly explained in European patentapplication EP 01201841.2. The intermediate generator matrix G″ islarger than the generator matrix G in that it comprises at least onemore column than the generator matrix G. In general, the generatormatrix G has k rows and n columns while the intermediate generatormatrix G″ has k rows and n+k columns and comprises k columns with asingle non-zero entry at mutually different positions. When using saidintermediate generator matrix G″ for encoding the information word m,intermediate codewords t having k+n symbols are obtained. From saidintermediate codeword t the codeword c is obtained from a codewordgenerating unit 44 by omitting a number of symbols of said intermediatecodeword t. Therein the number of symbols to omit corresponds to thedifference between the number of columns of said intermediate generatormatrix G″ and said generator matrix G. Thus, the obtained codeword ccomprises n symbols. However, it is to be noted that also G can be useddirectly for encoding in the encoding apparatus instead of G″.

During decoding a possibly multilated codeword r comprising n symbols isreceived by a decoder as shown in FIG. 4. In a first step the receivedword r is extended into a first pseudo codeword r′ by an extension unit50. Therein said intermediate generator matrix G″ which has already beenused in the encoder is used to determine the length of said pseudocodeword r′, i. e. the number of symbols of said pseudo codeword r′corresponds to the number of columns of said intermediate generatormatrix G″, i. e. to the n symbols of the received word r k erasures areadded to obtain the pseudo codeword r′. If G has been used directly forencoding instead of G″, the pseudo codeword r′ equals the n symbols ofthe received word r to which k erasures are added.

Thereafter, in a replacement unit 51 a priori known information symbols,e.g. m₁, m₅, m₆, are replaced in said pseudo codeword r′ at positions ofthe erasures which correspond to the positions of said a priori knowninformation symbols. This means that the erasures 1, 5 and 6 arereplaced by the a priori known information symbols m₁, m₅, m₆. Theobtained second pseudo codeword r″ is thereafter inputted to a decoderunit 52 which is preferably a known error and erasure decoder decodingsaid second pseudo codeword r″ by use of said intermediate generatormatrix G″ into the information word m comprising k symbols.

According to this embodiment a larger intermediate generator matrix G″is used compared to the standard generator matrix G. However, theadvantage of this embodiment is that the information symbols do not needto be known a priori in successive order but any additional informationsymbol known a priori irrespective of the position of the informationsymbol within the information word generally leads to an enhancedminimum Hamming distance compared to the code used if no informationsymbols are known a priori.

The embodiment based on code puncturing shall now be illustrateddifferently. Considered is an [8, 3, 6] extended Reed-Solomon Code Cover a Galois Field GF (8) defined as follows. The vector c=(c⁻¹, c₀, c₁. . . , c₆) is in C if and only if$c_{- 1} = {{\sum\limits_{i = 0}^{6}{c_{i}\quad{and}\quad{\sum\limits_{i = 0}^{6}{c_{i}\alpha^{ij}}}}} = {{0\quad{for}\quad 1} \leq j \leq 4.}}$Herein, α is an element of GF(8) satisfying α³=1+α.

It can be seen that the following intermediate generator matrix G″generates the code C $G^{''} = {\begin{pmatrix}1 & 0 & 0 & \alpha^{2} & 1 & \alpha^{6} & \alpha^{2} & \alpha^{6} \\0 & 1 & 0 & \alpha^{3} & 1 & \alpha^{3} & \alpha & \alpha \\0 & 0 & 1 & \alpha^{4} & 1 & \alpha^{5} & \alpha^{5} & \alpha^{4}\end{pmatrix}.}$The rightmost 5 columns of the intermediate generator matrix GC rareused as a generator matrix G, i. e. the generator matrix G is$G = {\begin{pmatrix}\alpha^{2} & 1 & \alpha^{6} & \alpha^{2} & \alpha^{6} \\\alpha^{3} & 1 & \alpha^{3} & \alpha & \alpha \\\alpha^{4} & 1 & \alpha^{5} & \alpha^{5} & \alpha^{4}\end{pmatrix}.}$The code generated by the generator matrix G has minimum Hammingdistance 3. Knowledge of any j information symbols effectively increasesthe minimum Hamming distance from 3 to 3+j.

Coming back to the present invention, in a first embodiment for use inDVR, as explained above with reference to FIG. 2 and as shown in FIG. 5,the codevector C may comprise Z5=16 user data symbols U (Z71=0) andZ6=16 parity symbols P1. In a second embodiment for use in DVR, as shownin FIG. 6, the codevector C may comprise Z4=32 parity symbols P but nouser data symbols U.

For decoding of the codevector C of the first embodiment (FIG. 5) 16erasures are put on the locations of the parity symbols P2 by thedecoder to reconstruct the codeword E, leaving Hamming distance 17available for correcting errors and erasures in the locations of theuser data symbols U and the parity symbols P in the codeword E.

For decoding of the codevector C of the second embodiment (FIG. 6) 16erasures are put on the locations of the user data symbols U by thedecoder to reconstruct the codeword E, again leaving at least Hammingdistance 17 available for correcting errors and erasures in thelocations of the user data symbols U and the parity symbols P in thecodeword E. However, if a number x of user data symbols are known apriori to the decoder, these need not be erased by the decoder enhancingthe remaining Hamming distance. Thus, the decoder decoding thereconstructed codeword E has Hamming distance 17+x available forcorrecting errors and erasures in the locations of the user data symbolsU and the parity symbols P in the codeword E.

User data symbols can, as an example described in European patentapplication EP 01201841.2, be known a priori to the decoder if much ofthe header information of a current sector can be inferred from thepreviously read sectors and the table of contents, or from the knowledgewhere the reading or writing head will approximately land. A possibleapplication is thus in the field of address retrieval on optical media.

It should be noted hat the encoding procedure of said second embodimentis similar to the embodiment described above with reference to FIGS. 3and 4. Therein a kx(n+k) matrix G″=(I,G) is used, where I is the k×kidentity matrix, and G a k×n generator matrix. Since the standard[62,30,33] RS code used according to the present invention is asystematic code, its 30×62 generator matrix G_(standadd) can be writtenas G_(standard)=(I,P′), where the 30×32 matrix P′ denotes the paritypart of the matrix G_(standard). Encoding of the dummy data symbols Dcorresponds to using the upper 14 rows of G_(standard), while encodingof the user data symbols U corresponds to using the lower 16 rows ofG_(standard). Because the dummy data symbols D are known at the decoder,it can be reconstructed free of errors at the decoder. Conceptually, thecontribution of the dummy data symbols D to the parities P is also knownat the decoder and can be subtracted from the parity symbols P to obtainintermediate parity symbols P″, which then only depend on the user datasymbols U.

The bottom 16 rows of G_(standard), form a 16×62 matrix of which thefirst 14 columns are all-zero. $G_{standard} = \begin{pmatrix}I_{14} & 0 & P_{14 \times 32}^{\prime} \\0 & I_{16} & P_{16 \times 32}^{\prime}\end{pmatrix}$The matrix I₁₆ corresponds to the systematic reproduction of the userdata symbols U in the codeword E which is not transmitted. The matrixP′_(16×32) corresponds to the part of the parity part P′ of G_(standard)that effectively generates the parities corresponding to the user datasymbols U. In terms of the embodiment shown in FIGS. 3 and 4, theequivalence is given by (I,G)=(I₁₆, P′_(16×32)).

It should be noted that the advantageous effect of using a number of apriori known user data symbols by the decoder can also be applied if thecodevector C is not formed exclusively by parity symbols as shown inFIG. 6, but also if the codevector C consists of a number of user datasymbols, but not all user data symbols, and a number of parity symbols.

It should be noted that the present invention is not limited to theabove-described embodiment or to encoding or decoding of data to bestored on a DVR information carrier. The invention is generallyapplicable in any kind of technical field where different kinds of datashall be encoded using more than one error correcting code havingdifferent numbers of parities, particularly in any new optical, magneticor mobile communication standard. The invention can also be applied toany kind of information carrier, be it a read-only, recordable orrewritable information carrier for storing any kind of data in any areaof such an information carrier. In addition, the codevectors need notnecessarily be stored but can also be transmitted over a network or atransmission line.

1-14. (canceled)
 15. A signal encoded in variations of physical properties of a medium, the signal being produced by the method of: generating a first block (B) of a fixed first number (Zi) of data symbols by taking a fixed second number (Z2), being smaller than said first number (Z1), of user data symbols (U), and a fixed third number (Z3) of dummy data symbols (D), and by arranging said user data symbols (U) and said dummy data symbols (D) in a predetermined order, encoding said first block (B) of data symbols using an ECC encoder (2) to obtain a codeword (E) having a fixed number of symbols, said codeword (E) comprising said first block (B) of data symbols and a second block of a fixed forth number (Z4) of parity symbols (P), and generating a codevector (C) by selecting a fifth predetermined number (Z5) of user data symbols (U2) and a sixth predetermined number (Z6) of parity symbols (P1) from said codeword (E), the sum of said fifth and sixth number being predetermined and smaller than the sum of said second and forth number.
 16. The signal of claim 15, wherein the medium is an optically readable information carrier.
 17. The signal of claim 15 wherein the medium is a computer network.
 18. An information carrier comprising a substrate with tracks encoded with a codevector (C) containing a fifth predetermined number (Z5) of user data symbols (U2) and a sixth predetermined number (Z6) of parity symbols (P1) from a codeword (E), the sum of said fifth and sixth number being predetermined and smaller than the sum of a fixed second number (Z2) and a fixed forth number (Z4); the codeword (E) having a fixed number of symbols, the codeword (E) comprising a first block (B) of data symbols and a second block of the fixed forth number (Z4) of parity symbols (P), the first block (B) of data symbols being ECC encoded (2); the first block (B) having the fixed first number (Z1) of data symbols, and the fixed second number (Z2), being smaller than said first number (Z1), of user data symbols (U), and a fixed third number (Z3) of dummy data symbols (D), and said user data symbols (U) and said dummy data symbols (D) being arranged in a predetermined order.
 19. A signal encoded in variations of physical properties of a medium, the signal comprising a codevector (C) containing a fifth predetermined number (Z5) of user data symbols (U2) and a sixth predetermined number (Z6) of parity symbols (P1) from a codeword (E), the sum of said fifth and sixth number being predetermined and smaller than the sum of a fixed second number (Z2) and a fixed forth number (Z4); the codeword (E) having a fixed number of symbols, the codeword (E) comprising a first block (B) of data symbols and a second block of the fixed forth number (Z4) of parity symbols (P), the first block (B) of data symbols being ECC encoded (2); the first block (B) having the fixed first number (Z1) of data symbols, and the fixed second number (Z2), being smaller than said first number (Z1), of user data symbols (U), and a fixed third number (Z3) of dummy data symbols (D), and said user data symbols (U) and said dummy data symbols (D) being arranged in a predetermined order. 